Hydrothermal treatment of kaolin clays to produce modified products of various types, has been known for many years, and is widely described in the prior art technical and patent literature. The reaction of sodium silicate with kaolin clays, for example, has been studied under various hydrothermal conditions, as reported by Kurbus et al, Z. Anogr. Allg. Chem., 1977, Volume 429, pages 156-161. These reactions were studied under hydrothermal conditions using essentially equivalent molar ratios of the kaolin and sodium silicate with the reaction being carried out in an autoclave. The products of the reactions, as identified by x-ray, electron microscope, and infrared methods, showed that sodium silicate reacts with kaolin to form an alumino-silica gel or a crystallized zeolite mineral analcime of the formula: EQU Na.sub.2 O:Al.sub.2 O.sub.3 :4SiO.sub.2 2H.sub.2 O
In the reaction, the kaolin dissolves and alpha-quartz simultaneously appears in the product of reaction.
U.S. Pat. No. 4,812,299 issued Mar. 14, 1989 to S.K. Wason (assigned to J.M. Huber Corp.) describes compositions designated as synthetic alkali metal alumino-silicates, or simply SAMS, prepared by the hydrothermal reaction of an alkali metal silicate and kaolin clay. By the term "hydrothermal" it is meant that the reaction is carried out under aqueous conditions of elevated temperatures and pressures of greater than atmospheric. The reaction is conducted under conditions of agitation. In a preferred operation of the said process, the aqueous slurry of the starting clay material and the alkali metal silicate is formed, the system is closed and heat applied to gradually raise the temperature. In general, the pressure in the system will range from about 50 to 360 psig at temperatures ranging from about 140 to 250 degrees C. A specifically preferred range of conditions is to operate the process at pressures of 100 to 200 psig and temperatures of 164.degree. to 194.degree. degrees C. The temperatures are preferably correlated to the pressure such as that provided by steam. The reaction time is about 0.25 to 4 hours. After completion of the reaction, heat is removed and the mixture is allowed to cool, after which the system is opened, the product separated by filtration or centrifugation, washed with water, and dried. Spray drying is preferred at inlet temperatures of 1000.degree. F. (538.degree. C.) and outlet temperature of 250.degree. F. (121.degree. C ). The products are defined as structured agglomerates wherein the primary particles comprise altered kaolin clay platelets integrated with one or more adjacent areas of essentially amorphous alkali metal silicate base-kaolin clay reaction products. More specifically, they are described as altered kaolin platelets having an integrated rimmed area of amorphous, non-diffracting alkali metal silicate-kaolin reaction product. The products are said to be useful as reinforcing agents or fillers for papers, paints, plastics and rubber and to have increased opacity and brightness as compared with the starting clay material.
U.S. Pat. No. 4,816,074 issued Mar. 28, 1989 to Rasik H. Raythatha et al, assigned to E.C.C. America Inc., describes a non-hydrothermal process in which a structured aggregated kaolin pigment is prepared by mixing substantially dry kaolin in particulate form with an aqueous alkali metal silicate to deposit on the surface of the kaolin particles a substantially molecular level of said silicate without formation of silica gel, drying the treated kaolin without calcination and exposing it to an acidic gas. The product is useful as a pigment in the coating or filling of paper. The process serves to aggregate the very fine particles (i.e. the sub 0.25 micrometer particles) present in very fine feeds. Thus, the otherwise large percentages of troublesome extremely fine particles are effectively removed as separate entities, but without the need for separation steps, together with the costly equipment required for such operations.
U.S. Pat. No. 3,765,825 issued Oct. 16, 1973 to V.J. Hurst teaches reducing the viscosity of kaolin clay slurries by hydrothermal treatment. In U.S. Pat. No. 3,769,383 issued Oct. 30, 1973, he teaches hydrothermal treatment of kaolin to produce a variety of alumino-silicate products.
U.S. Pat. No. 4,499,062 issued Feb. 12, 1985 to P. Christophliemk et al (assigned to Henkel) teaches hydrothermal decomposition of industrial waste products such as filter sludge containing aluminum silicate and/or alkali aluminum silicate, by addition of aqueous NaOH solution and sand.
In pending patent application Ser. No. 416,936 of Dunaway et al. filed Oct. 4, 1989, which application is assigned to the assignee of the instant application, there is disclosed a method which enables relatively low temperature aggregation of kaolin, which can be hydrothermal. In the practice of the Dunaway invention an aqueous slurry of particulate kaolin clay and sodium aluminate is prepared, having a total solids concentration of from about 2 to 25% by weight, the weight ratio between kaolin and aluminate being greater than 1. The slurry is then treated in a closed system to form aggregates of adherent kaolin platelets. The treatment is generally effected at temperatures of from about 50.degree. to 300.degree. C., with a more preferable temperature being in the range of from about 90.degree. to 200.degree. C. Pressure conditions in the closed system correspond to saturated steam at the temperature utilized. The slurry is subjected to mixing during the hydrothermal treatment, with such treatment being carried out for from about 0.1 to 8 hours; and preferably for from about 0.1 to 2 hours. In a further aspect of the Dunaway et al invention, it has been found that generally lower temperatures can be used during the hydrothermal reaction, where the slurry further includes sodium silicate. Where so used, the molar ratio between the sodium aluminate (expressed as equivalent Al.sub.2 O.sub.3) and sodium silicate (expressed as equivalent SiO.sub.2) is from about 1:2 to 2:1. Following the hydrothermal reaction, the resultant aggregates are recovered and dried. Wide varieties of clays can be processed including inferior, low grade clays which may be unsuitable for calcination. Discolored clays having low brightnesses can also be used as feed for the process, and unexpectedly brightened pigments are thereby yielded.
In Swift, U.S. Pat. No. 3,849,149, a method is disclosed for coating mineral particles which can include alumino silicates. The purpose of the coating on the mineral particles is to render the surface acidic so that the coating materials enter into a polymerization reaction with basic organic materials. Since the objective is not aggregation, the quantity of additives is relatively low, and it is important that the particles remain separate so that each particle can be coated in the subsequent polymerization reaction.
In Hanahan, U.S. Pat. No. 2,296,637, a high-surface hiding pigment material and process of making same is disclosed. In one aspect of the method of preparation, a sodium silicate solution is added to a clay pigment slurry and is precipitated upon the clay surface by creating an acid environment such as by adding sulfuric acid.
In Hanahan, U.S. Pat. No, 2,296,639, similar materials to those described in the 2,296,637 patent are precipitated, especially upon lithopone. The patent also indicates that the process can be used with other extender materials such as aluminum silicates. The reaction product formed can be that of sodium silicate and aluminum sulfate. The preferred amounts are very low (0.35%-1.5% silicate). Percentages higher than 10% are said by the patentee to result in decreased surface hiding power when the pigments are used in paints.
In Kurrle, U.S. Pat. No. 4,026,721, a composite silicate pigment is prepared by a precipitation reaction wherein spherical hydrous metal silicate particles are precipitated on the planar surfaces of clay particles having a platelet-type structure. An aqueous suspension of the clay pigment is formed, and a water soluble salt of an alkaline earth metal is blended into the clay slurry. Under high shear, a water soluble alkali metal silicate is then metered into the slurry to precipitate the alkaline earth metal silicate (preferably calcium silicate) in the form of spherical particles on the clay platelets. The composite pigment is then filtered and washed. The clay component may comprise kaolinite. The small spherical particles on the clay platelets are stated by the patentee to not significantly alter the overall particle size distribution of the base material.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide a process for aggregating a kaolin feed, which process can be effectively practiced at substantially room temperature (and pressure) and which result in an aggregated pigment displaying good brightness and high pore volume, thereby rendering such pigment highly suitable for paper coating and paper filling applications.